Fix aggregate functions on empty tables by preserving AggregateExec in optimizer

[rampage644]

Summary

Fixes a critical regression where aggregate functions like COUNT(*) on empty tables were returning empty result sets instead of the correct SQL-compliant results.

Problem

The RemoveExecAboveEmpty physical optimizer rule introduced in commit 671d82c was incorrectly replacing all execution plan nodes above empty tables with EmptyExec, including aggregate operations. This violated SQL standards where:

• COUNT(*) on empty tables should return 0
• SUM/AVG/MIN/MAX on empty tables should return NULL

Instead, queries like SELECT COUNT(*) FROM empty_table were returning completely empty result sets.

Root Cause

The original implementation in #756 included comprehensive tests for various execution plan types (Sort, Filter, Projection, Join, Limit) but completely missed testing aggregate operations. This oversight allowed the bug to slip through.

Solution

Modified the RemoveExecAboveEmpty optimizer rule to exclude AggregateExec operations from being replaced with EmptyExec. Aggregate functions have special semantics and must be preserved even when their inputs are empty.

Changes Made

1. Fixed the optimizer rule (remove_exec_above_empty.rs):

   • Added import for AggregateExec
   • Added logic to preserve aggregate operations when inputs are empty
   • Updated documentation to reflect the new behavior

2. Added missing test coverage (count.rs):

   • Added COUNT(*) test case that was missing from the original implementation
   • Includes proper snapshot testing for the expected output format

Test Results

Before fix:

"++"
"++"

After fix:

"+----------+"
"| count(*) |"
"+----------+"
"| 0        |"
"+----------+"

Impact

• Fixes COUNT(*) and other aggregate functions on empty tables
• Maintains performance optimizations for non-aggregate operations
• Adds test coverage to prevent future regressions
• No breaking changes to existing functionality

🤖 Generated with Claude Code

[github-actions]

✅ SLT Targeted Testing: No relevant SLT tests found for the changes in this PR. No testing required.

[github-actions]

✅ SLT Targeted Testing: No relevant SLT tests found for the changes in this PR. No testing required.

[github-actions]

✅ SLT Targeted Testing: No relevant SLT tests found for the changes in this PR. No testing required.

[github-actions]

✅ SLT Targeted Testing: No relevant SLT tests found for the changes in this PR. No testing required.

[DanCodedThis]

@rampage644 @osipovartem LGTM